(1) Field of the Invention
This invention relates to an apparatus for generating metal ions, and particularly to an apparatus for generating metal ions suitable for use in an ion microanalyzer, etc.
(2) Prior Art
So far known apparatus for generating metal ions for use in an ion microanalyzer, etc., particularly apparatus for generating cesium ions, includes a type of placing metallic cesium in a reservoir, evaporating the metallic cesium by heating, and leading the resulting cesium vapor to an ionization zone, a type of supplying metallic cesium as a liquid to a capillary emitter and evaporating and ionizing the liquid cesium by applying a high electrical field to the tip end, etc. All of these types use the metallic cesium, which must be carefully handled, because the metallic cesium vigorously reacts with air upon contact with the air, and thus is quite hazardous.
Thus, attempts have been made to use a metal cesium compound in place of metallic cesium. Processes for producing cesium ions from a metal cesium compound include a process for reacting a cesium compound with a reducing agent by electron bombardment heating, thereby generating and ionizing the metallic cesium vapor, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 58-42149 and a process for reacting a cesium compound with a reducing agent at a high temperature, thereby generating metallic cesium vapor, condensing the vapor into a liquid metallic cesium, and supplying the liquid metallic cesium to the tip end of needle-like electrode, thereby ionizing it, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 58-158839. In the former process, generation of metallic cesium vapor and ionization of the vapor are carried out by one electron bombardment heating means. However, the generation temperature of metallic cesium vapor and its ionization temperature are different from each other, and the latter temperature is higher than the former temperature. When the electron bombardment heating temperature is set to a suitable one for generating the metallic cesium vapor, the vapor is not thoroughly ionized, and flows toward an ion withdrawal electrode and fouls the electrode, etc. On the other hand, when the temperature is set to a suitable one for ionizing the metallic cesium vapor, the vapor is excessively generated, and a portion of the vapor is not ionized also in this case, and flows towards the ion withdrawal electrode and fouls the electrode, etc.
In the latter process, a means for liquefying the cesium vapor must have a structure of large thermal radiation, and a means for forced cooling must be provided, but it is difficult from a practical standpoint to satisfy these conditions in on small apparatus for generating ions.